OSS STS鉴权模式的Token更新问题

id credential = [[OSSFederationCredentialProvider alloc] initWithFederationTokenGetter:^OSSFederationToken * {    NSURL * url = [NSURL URLWithString:[NSString stringWithFormat:@ "%@%@",Base_API,GetSTSToken_API]];NSMutableURLRequest *request=[NSMutableURLRequest requestWithURL:url]; //默认为get请求request.timeoutInterval= 10.0; //设置请求超时为10秒request.HTTPMethod=@ "POST"; //设置请求方法OSSTaskCompletionSource * tcs = [OSSTaskCompletionSource taskCompletionSource];NSURLSession * session = [NSURLSession sharedSession];NSURLSessionTask * sessionTask = [session dataTaskWithRequest:requestcompletionHandler:^(NSData *data, NSURLResponse *response, NSError *error) { if (error) {[tcs setError:error]; return;}[tcs setResult:data];}];[sessionTask resume];[tcs.task waitUntilFinished]; if (tcs.task.error) {
// NSLog(@"get token error: %@", tcs.task.error);return nil;} else {NSDictionary * object = [NSJSONSerialization JSONObjectWithData:tcs.task.resultoptions:kNilOptionserror:nil];OSSFederationToken * token = [OSSFederationToken new];token.tAccessKey = [object[@"Credentials"] objectForKey:@"AccessKeyId"];token.tSecretKey = [object[@"Credentials"] objectForKey:@"AccessKeySecret"];token.tToken = [object[@"Credentials"] objectForKey:@"SecurityToken"];token.expirationTimeInGMTFormat = [object[@"Credentials"] objectForKey:@"Expiration"];return token;}}];
文档上说能根据过期时间自动更新token，但是我发现这个block每次都会执行，不是应该到了过期时间才会执行吗？不知道是什么原因，请大家帮帮忙。。。
是过期时间传递的不对吗？过期时间是阿里接口返回的。。。